Xcode generator doesn't respect SYSTEM when setting include_directories

When using target_include_directories(target SYSTEM <paths...>), the Make and Ninja generators use -isystem to pass the include directories to the compiler. Xcode has the properties HEADER_SEARCH_PATHS (which get passed with -I to the compiler), and SYSTEM_HEADER_SEARCH_PATHS, which uses -isystem. However, the Xcode generator indiscriminately passes all paths into the HEADER_SEARCH_PATHS.